My chimney stack has just been taken down below the roof line and covered over (capped) in the loft, when my roof was re tiled. We live in a detetched house with the chimney in question next to the outside wall.
Do I need to vent the chimney? If so I would have to put a vent low down in the fire place at ground floor level, but the thing I am unsure of is where to place another vent to allow air flow.
I dont really want to vent the air into my loft space as I am worried about the warmer air condensing. I cant put an air brick outside near the roof line as access is very difficult. Would there be any benefit of fitting a vent in the upstairs bedroom near the ceiling by drilling through the side of the chimney breast.

It rather depends on what type of home you have.
If you have an open cavity wall, then the single leaf brick wall that makes up the breast of the chimney will be the same as the rest of the home.=Do nothing.

If you have a filled cavity or solid walls then the single brick chimney breast is the weak link in the home.
It has roughly the same insulation value as a double glazed window.
This means that if your windows steam up when its cold......then there is condensation landing and moving into your chimney breast. (that you cannot see, until you get a damp patch)

Where you can wipe the window dry, the damp moves into the chimney wall, makes it damp.

Water is a better conductor than air about 4000 times better.

This means a damp wall is removing the heat from your home at an expensive rate.

Does the room feel cold and damp?

If it does then the best solution is to remove the rest of the chimney.

Or, you can fix one inch polystyrene over the chimney breast to improve the insulation and re direct the water vapour to the window.

The problem is that where you have a gap larger than 16mm the air in the gap starts to rotate as a convection current, so that 20cm gap in the chimney quickly moves the warm air from your room, across the chimney and out to the sky.